Disc evolution processes: how they affect the inner disc

2018-11-16T05:58:58Z (GMT) by Rosotti, Giovanni
Most planets discovered by Kepler are super-Earths with small gaseous atmospheres, showing that they formed in a gas-rich environment. In this review I will discuss the three main processes affecting the gas in the inner disc: accretion, photo-evaporation, and the formation of giant planets. In the conventional view of proto-planetary discs, the inner disc is "fed" by the outer disc where most of the mass resides and its evolution is intimately linked to the overall evolution of the disc. Alternatively, large mass reservoirs close to the star are required, predicting much higher densities for the formation environment of the Kepler-discovered planets. I will discuss the current observational constraints on this topic, provided by combining different indicators tracing the inner (accretion onto the star) and outer (ALMA observations) disc for large samples of stars. The two main candidates for the accretion mechanism are angular momentum transport by viscosity and angular momentum removal by winds. I will discuss how the properties of the inner disc change depending on which dominates, with important implications for the location of the water snow-line and planetary migration. The inner disc is also subject to the photo-evaporative wind. With time, this depletes the inner disc of material (forming a so-called transition disc), ultimately setting the timescale available for super-Earths to assemble their gaseous atmospheres. The last piece in the puzzle to describe the evolution of the inner disc is giant planet formation. Giant planets affect how much mass gets in the inner disc by cutting the supply of material to the inner disc, in this way hastening the dispersal of the inner disc; in turn their migration is affected by how much material is left in the disc.